In recent years the attention for a display cell having a 180.degree.-360.degree. twist has grown steadily. This applies more particularly to a display cell having a 180.degree.-270.degree. twist and more notably to one having a 270.degree. twist. During operation the cell is exposed to linearly polarised white light which is incident via the transparent substrate plate. During its passage through the cell the linearly polarised light is changed into elliptically polarised light due to birefringence of the cell medium and the twist configuration. The cell having a 180.degree.-360.degree. twist is known as STN cell, i.e. supertwist nematic cell. The special STN cell having a 270.degree. twisted configuration is known as supertwist birefringence effect (SBE) cell.
The advantage of this type of cell relative to, in particular, the well-known twisted nematic cell (TN cell) having a 90.degree. twist is the steep transmission-voltage characteristic. This means that when a voltage is applied across the cell, a large difference in transmission of the light through the cell occurs in a very small voltage range around a threshold value. Thus, it becomes possible to control a large number of lines by means of time multiplexing, yielding a high information density.
The STN or SBE cell also has disadvantages. The first disadvantage being that two polarisers are required. The cell is operated only with linearly polarised light. When no voltage or a voltage below the threshold voltage is applied across the electrodes of the cell, the director exhibits the above-mentioned 180.degree.-360.degree. twist and preferably a 270.degree. twist. The cell is exposed to linearly polarised light via the polariser, which light passes through the cell via the transparent substrate plate and the transparent electrode in a direction perpendicular to the substrate plates. Due to birefringence the linearly polarised light is changed into elliptically polarised light. This change depends on the wavelength. At the analyser connected to the other substrate plate a spectrum is obtained; a coloured effect. Which spectrum--more in particular which colour effect--is obtained depends on several factors. One important factor being the selected position of the analyser relative to the polariser. The type of liquid crystalline material also plays a part. In fact the analyser provides a cross-section through the various ellipses. In the case of the SBE cell this results, for example, in a blue-white contrast or a yellow-black contrast for parallel and crossed polarisers, respectively.
When a voltage equal to or larger than the threshold voltage is applied across the cell thickness by means of the electrodes, the molecules of the liquid crystalline cell medium are oriented in conformity with the field lines. This causes the twist to disappear and a situation of optical isotropy to occur. The light transmission changes. This causes the above-stated contrast of a SBE cell.
A second important disadvantage of the STN (SBE) cell is that it is not suitable for use as a colour display cell in, for example, a colour TV. A first requirement to be met by a colour display cell is that black-white operation must be possible, as in the case of a TN cell having a 90.degree. twist. When the TN cell is in the non-energised state, white linearly polarised light is passed when the polariser and the analyser intersect, such that each colour can be obtained by means of colour filters. In the energised state, and the polariser and analyser intersecting, the white linearly polarised light is fully absorbed. However, a TN cell has the disadvantage that the transmission-voltage characteristic is less steep.
It has been tried to obtain or substantially obtain a black-white effect in a SBE cell by adding a colourant which absorbs the dominating spectral component. It has been found, however, that such large concentrations of colourant are needed that it adversely affects the transmission-voltage characteristic.